From a functional standpoint, the humerus is a key element in the skeleton of vertebrates as it is the forelimb’s bone that connects with the pectoral girdle. In most birds, the humerus receives both the forces exerted by the main flight muscles and the aerodynamical stresses exerted upon the wing during locomotion. Despite this functional preeminence, broad scale studies of the morphological disparity of the humerus in the crown group of birds (Neornithes) are lacking. Here, we explore the variation in shape of the humeral outline in modern birds and its evolutionary relationship with size and the evolution of different functional regimes, including several flight strategies, wing propelled diving and complete loss of wing locomotory function. Our findings suggest that most neornithines evolved repeatedly towards a general humeral morphology linked with functional advantages related with more efficient flapping. Lineages evolving high-stress locomotion such as hyperaeriality (e.g., swifts), hovering (e.g., hummingbirds) and wing-propelled diving (e.g., penguins) greatly deviate from this general trend, each exploring different morphologies. Secondarily flightless birds deviate to a lesser degree from their parent clades in humeral morphology likely as a result of the release from constraints related with wing-based locomotion. Furthermore, these taxa show a different allometric trend that flighted birds. Our results reveal that the constraints of aerial and aquatic locomotion are main factors shaping the macroevolution of humeral morphology in modern birds.

中文翻译：

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现代鸟类的肱骨形态学差异

从功能的角度来看，肱骨是脊椎动物骨骼中的关键元素，因为它是与胸带相连的前肢骨骼。在大多数鸟类中，肱骨既承受主要飞行肌肉施加的力，又承受运动过程中施加在机翼上的空气动力应力。尽管具有这种功能上的优势，但仍缺乏对鸟类冠状群（Neornithes）中肱骨形态差异的广泛研究。在这里，我们探讨了现代鸟类的肱骨轮廓形状的变化及其与大小的进化关系以及不同功能模式的演变，包括几种飞行策略，机翼推进式潜水和机翼机能的完全丧失。我们的发现表明，大多数神经鸟氨酸反复向着普通的肱骨形态发展，并具有与更有效拍打相关的功能优势。演变为高压力运动的血统，例如高空度（例如飞跃），盘旋（例如蜂鸟）和有翼推进的潜水（例如企鹅），都与这种总体趋势大相径庭，每一个都探索不同的形态。其次，可能是由于与基于机翼的运动相关的约束的释放而导致无法飞行的鸟类在肱骨形态上偏离其父本进化枝的程度较小。此外，这些分类单元显示出与飞鸟不同的异速变化趋势。我们的研究结果表明，空中和水上运动的约束是塑造现代鸟类肱骨形态宏观进化的主要因素。